Round up the usual suspects. In any US presidential election campaign, you can be sure that protectionism will break in, as it has. But America's swing back toward protectionism started long before, with President George W. Bush's steel tariff of 2001.
It was hard to understand what the underlying calculus was when the Bush administration imposed that tariff on steel imports. The tariff was bad economics: it made America poorer. The tariff was bad mercantilism: it robbed more in profits and unionized jobs from steel-consuming industries than it gave to steel-making industries, and the former were at least as well-organized and vocal in Washington as the latter.